Edwin then succeeded to the Northumbrian throne, driving out the sons of Æthelfrith. We have little evidence of Edwin's activity outside of Northumbria before 625. It is probable that the conquest of the Celtic kingdom of Elmet[?], located in the neighbourhood of the modern Leeds, ruled over by a king named Cerdic[?] fell within this period, and this may have led to the later quarrel with Cadwallon[?], king of Gwynedd.

Edwin seems also to have annexed Lindsey to his kingdom by 625. In this year he entered negotiations with Eadbald of Kent to marry his sister Ælthelberg. It was a condition that Christianity should be tolerated in Northumbria, and accordingly Paulinus was consecrated bishop by Justus in 625, and was sent to Northumbria with Æthelberg.

According to Bede, Edwin was favourably disposed towards Christianity owing to a vision he had seen at the court of Raedwald, and in 626 he allowed Eanfled, his daughter by Æthelberg, to be baptized. On the day of the birth of his daughter, an attempt was made on the king's life by Eomer[?], an emissary of Cwichelm[?], king of Wessex. Saved by the devotion of his thegn Lilla, Edwin vowed to become a Christian if victorious over his treacherous enemy. He was successful in the ensuing campaign, and gave up worshipping his traditional gods.